1. Field of the Invention
The invention generally relates to a method of obtaining para-xylene from a feed of C8 aromatics comprising xylene isomers.
2. Brief Description of Related Technology
Hydrocarbon mixtures containing C8+ aromatics are by-products of certain oil refinery processes including, but not limited to, catalytic reforming processes. These hydrocarbon mixtures typically contain up to about 30 weight percent (wt. %) C9+ aromatics, up to about 10 wt. % non-aromatics, up to about 50 wt. % ethylbenzene, the balance (e.g., up to about 100 wt. %) being a mixture of xylene isomers. Most commonly present among the C8 aromatics are ethylbenzene (“EB”), and xylene isomers, including meta-xylene (“mX”), ortho-xylene (“oX”), and para-xylene (“pX”). Typically, when present among the C8 aromatics, ethylbenzene is present in a concentration of up to about 20 wt. % based on the total weight of the C8 aromatics. The three xylene isomers typically comprise the remainder of the C8 aromatics, and are present at an equilibrium weight ratio of about 1:2:1 (oX:mX:pX). Thus, as used herein, the term “equilibrated mixture of xylene isomers” refers to a mixture containing the isomers in the weight ratio of about 1:2:1 (oX:mX:pX).
Ethylbenzene is useful in making styrene. Meta-xylene is useful in making isophthalic acid, which itself is useful to make specialty polyester fibers, paints, and resins. Ortho-xylene is useful in making phthalic anhydride, which itself is useful to make phthalate-based plasticizers. While meta-xylene and ortho-xylene are useful raw materials, demands for these isomers and materials made therefrom are not as great as the demand for para-xylene and the materials made from para-xylene. Para-xylene is a raw material useful in making terephthalic acids and esters, which are used to make polymers, such as poly(butene terephthalate), poly(ethylene terephthalate), and poly(propylene terephthalate).
Because of their usefulness, efficient separation of ethylbenzene and the various xylene isomers from each other is of particular and continuing interest. Depending upon the concentrations in which each is present in a C8 aromatics mixture, and depending upon the demand of a particular isomer over the others or ethylbenzene or a material made therefrom, separation alone may not be sufficient to obtain adequate quantities of any particular isomer. For example, because there is generally a higher demand for para-xylene compared to its other isomers or ethylbenzene, it is usually more desirable to increase or even maximize para-xylene production from a particular C8 aromatics mixture. Thus, separation of para-xylene oftentimes is coupled with isomerization of meta- and ortho-xylene isomers to the desired para-xylene and, optionally, conversion of the ethylbenzene.
The separation step of para-xylene production processes generally falls into two categories, one of which is crystallization and the other of which is liquid-phase adsorption chromatography. Crystallization was initially developed by Amoco Corporation with subsequent improvements and modifications by Institut Francais du Petrole (“IFP”), Mobil Corporation (“Mobil”), UOP Inc. (“UOP”), and others. As described in more detail below, crystallization has its limits and can be very expensive since crystallization of the various xylene isomers occurs at very low temperatures (e.g., about −70° C. to 0° C.), typically requiring multi-stage refrigeration systems with large gas compressors. Liquid-phase adsorption chromatography, also referred to as simulated moving bed adsorption chromatography (“SiMBAC”), was commercially developed by IFP and UOP. SiMBAC also has its limits and is expensive to operate because it requires a large-volume internal recycle of various hydrocarbon desorbent materials. Additionally, the effluent streams from the adsorption step must be separated from the desirable products in downstream distillation steps. Thus, the foregoing conventional crystallization and liquid-phase adsorption chromatography processes are disadvantageous because of significant capital and energy costs associated with each.
One method of producing para-xylene from a C8+ hydrocarbon mixture includes passing the mixture through a separation column to remove heavies, such as C9+ hydrocarbons. A lighter, overhead stream from the column, predominantly containing a C8 hydrocarbon mixture comprising the xylene isomers and ethylbenzene, can be resolved in a separation unit. Because ethylbenzene, meta-, ortho-, and para-xylenes have identical molecular weights and have similar boiling points (of about 136° C., about 139° C., about 138° C., and about 144° C., respectively), separation by way of fractional distillation is impractical. An alternative to fractional distillation includes low-temperature crystallization, which exploits the differences in freezing or crystallization temperatures of the various components—para-xylene crystallizes (at about 13.3° C.) before the other xylene isomers (ortho-xylene and meta-xylene crystallize at about −25.2° C. and about −47.9° C., respectively). In the physical system of the three xylene isomers, there are two binary eutectics of importance: the para-xylene/meta-xylene binary eutectic and the para-xylene/ortho-xylene binary eutectic. As para-xylene crystallizes from the mixture, the remaining mixture approaches one of these binary eutectics, depending upon the starting composition of the mixture. Therefore, in commercial-scale processes, para-xylene is crystallized such that the binary eutectics are approached—but not reached—to avoid co-crystallization of the other xylene isomers, which would lower the purity of the obtained para-xylene. Because of these binary eutectics, the amount of para-xylene recoverable per pass through a crystallization process typically is no greater than about 65% of the amount of para-xylene present in the stream fed to the crystallization unit.
Alternatively, certain components of the C8 hydrocarbon mixture may be separated from the mixture prior to any crystallization such as, for example, by liquid phase adsorption (e.g., UOP's PAREX™ process and IFP's ELUXYL™ process) utilizing a faujasite (zeolite) to chromatographically separate para-xylene from C8 mixtures containing para-xylene. The para-xylene lean stream exiting the separation unit typically is pressurized and reacted in the presence of a catalyst to obtain an equilibrated mixture of xylene isomers, which is then recycled to the liquid adsorber. By separating para-xylene from the C8 hydrocarbon mixture prior to crystallization, para-xylene recovery in the crystallization unit can be increased from no greater than about 65% to about 85%, overcoming some of the problems posed by the binary eutectics. See generally Swift (UOP) U.S. Pat. No. 5,329,060.
Another method of producing para-xylene from a C8 hydrocarbon mixture includes passing the mixture, in a gaseous phase, through an adsorption bed containing an adsorbent that is selective for adsorbing para-xylene and ethylbenzene to obtain, after suitable desorption, separate streams, one of which contains predominantly meta- and ortho-xylenes and the other of which contains para-xylene, ethylbenzene, and desorbed feed present in void spaces of the adsorbent. The adsorption is carried out at a temperature between 140° C. and 370° C., a pressure between atmospheric pressure and 300 kilopascals (absolute) (kPa) (about 65 pounds per square inch absolute (psia)), and with the aid of Mobil-5 (MFI) type zeolite molecular sieves, including ZSM-5 (Zeolite Socony Mobil zeolite molecular sieves are commercially available from ExxonMobil Chemicals), ferrierite, and silicalite-1 zeolite molecular sieves, including binder free silicalite-1 zeolite molecular sieves. The desorption of the para-xylene and ethylbenzene can be carried out with a gaseous desorbent containing water at a temperature within the same range of the adsorption, and at a pressure between atmospheric and 1000 kPa (about 145 psia). Alternatively, the desorption can be carried out without a desorbent, by mere depressurization at a pressure between 1 kPa and 4 kPa (about 0.15 psia to about 0.58 psia). In such a method, a substantial amount of feed remains in the voids of the adsorbent, which eventually is removed during the desorption step, but disadvantageously contaminates the desorbed stream containing para-xylene and ethylbenzene. See generally Long et al. (China Petrochemical Company and Fudan University) Chinese Patent Publication No. 1,136,549 A. Others have used ZSM-8 and ZSM-5 (each optionally reacted with silanes) to separate aromatics like xylene isomers and ethylbenzene such as disclosed in, for example, U.S. Pat. Nos. 3,699,182, 3,729,523, and 4,705, 909, and British Patent No. 1,420,796.
The foregoing crystallization and SiMBAC steps can be made more attractive if the feedstocks to those steps were re-formulated to contain a higher-than-equilibrium concentration of para-xylene. Such a re-formulation can be carried out by selective toluene disproportionation as described in, for example, International (PCT) Publication Nos. WO 00/69796 and WO 93/17987. Both crystallization and SiMBAC steps also can be designed and operated by those skilled in the art to concentrate para-xylene streams for subsequent purification steps. However, even these steps suffer from many (or all) of the disadvantages discussed above. Another method to produce feedstocks with higher-than-equilibrium para-xylene concentration is by pressure swing adsorption (“PSA”) processes. Such processes have been widely practiced for separation of gases such as air into nitrogen and oxygen, water removal from air, and hydrogen purification, and are generally described in Ralph T. Yang, “Gas Separation by Adsorption Processes,” pp. 237-274 (Butterworth Publishers, Boston, 1987) (TP242.Y36).
Fewer applications of PSA have been realized for hydrocarbon purifications, particularly purification of hydrocarbons that are liquid under ambient conditions, however. One notable exception is the ISOSIV™ process, developed by the Union Carbide Corporation, which is useful for the separation of straight-chain or normal paraffinic hydrocarbons from branched or iso-paraffinic hydrocarbons. The ISOSIV™ process operates at substantially constant adsorption pressure (i.e., constant total pressure in the adsorption unit during the adsorption step) and uses an inert gas (e.g., hydrogen) to purge, sweep, or otherwise achieve desorption of the adsorbed hydrocarbons from the adsorbent. See U.S. Pat. No. 3,700,589. Over time, the ISOSIV™ process was improved by adding additional adsorption units in the adsorption step, thus permitting recycle of feed to improve the overall purity and recovery of the desired iso-paraffin. See U.S. Pat. No. 4,176,053. The use of multiple adsorption units during the adsorption step, however, has its limits as diminishing returns are realized when too many units are used. See U.S. Pat. Nos. 4,476,345 and 4,595,490 (disclosing the benefits of fewer units and staggered adsorption/desorption cycles). The ISOSIV™ process can be made more attractive if the paraffin mixture fed to the adsorption unit has a higher-than-equilibrium concentration of n-paraffins. Thus, U.S. Pat. No. 4,210,771 discloses an isomerization unit upstream of the adsorption unit to convert iso-paraffins to n-paraffins. In the ISOSIV™ processes utilizing a downstream isomerization reactor (downstream relative to the adsorption unit), it is the adsorbed material that is desorbed and ultimately isomerized—the raffinate from the adsorber bed is not isomerized. Other improvements have been made to the ISOSIV™ process in general. See e.g., U.S. Pat. Nos. 4,372,022 and 4,709,117. Notably, however, no provisions are made in the ISOSIV™ process to separate hydrocarbons other than paraffins.
Clearly ISOSIV™ process effluent streams require capital- and energy-intensive downstream equipment and processing. Moreover, application of the ISOSIV™ process and the associated teachings to the production of para-xylene would present its own problems not adequately addressed in the art. For example, the para-xylene-lean effluent will require expensive pressurization (or re-pressurization) as it is fed to a downstream isomerization unit. As noted above, separation of xylene isomers oftentimes is coupled with isomerization of meta- and ortho-xylenes to the desired para-xylene. Such isomerization is carried out in a high pressure reactor. Even with an understanding of the ISOSIV™ process and an attempted application of the process to produce para-xylene, there is no guidance as to how to introduce the para-xylene depleted raffinate to the isomerization reactor without an expensive pressurization (or repressurization) step between the adsorption unit and the isomerization reactor.
Deckman et al. (Exxon Chemical Company) U.S. Patent Application Publication No. 2002/0065444 A1, for example, discloses a method of making para-xylene from mixed xylenes specifying a PSA or temperature-swing adsorption (“TSA”) unit and at least one isomerization reactor. The Deckman publication teaches an isomerization reactor immediately upstream of the PSA unit and no compression step between the reactor and the PSA unit. See FIGS. 1 and 2 of the Deckman publication. Such a teaching implies to one skilled in the art that the reactor operates and reactor effluents exit the reactor at a pressure exceeding the inlet pressure of the PSA unit. The raffinate exits the PSA unit at a high temperature, but reduced pressure (due to depressurization to accomplish desorption), requiring a compressor (or blower) to pressurize the stream before it is recycled to the isomerization reactor. Though not expressly described in the Deckman publication, the raffinate exits the PSA at temperatures too high to make compression practical in a compressor. Thus, the raffinate must be cooled to a suitable temperature before it can be compressed. In cooling the raffinate, certain constituents therein (e.g., xylene isomers) will condense. All of the condensable material must be separated from the uncondensable gas. The condensed material then is heated and pumped into the isomerization reactor, while the now-cooled, uncondensable gas is compressed in a compressor and then sent into the isomerization reactor. Thus, the disclosure in the Deckman publication is somewhat incomplete in that it lacks a specific disclosure of the necessary heat exchanger (condenser), gas/liquid separator, and liquids pump required to pass the PSA raffinate into the isomerization reactor at the appropriate pressure. Notwithstanding, the disclosed method requires a blower or compressor on each of the swing adsorption process effluent streams. As readily understood by those skilled in the art, such blowers and compressors typically are very expensive to purchase and operate, and should be avoided whenever possible.
One might consider operating the swing adsorption unit disclosed in the Deckman publication at a pressure high enough such that the reduced pressure to accomplish desorption remains sufficiently high so as not to require additional pressurization prior to feeding to downstream processing units such as an isomerization reactor. In practice, however, operating the adsorption unit at such a high desorption pressure will disadvantageously and dramatically lower the productivity of the adsorbent.
Thus, while there are various methods of obtaining para-xylene from a C8 aromatic mixtures, these methods are very complex and include necessary and expensive upstream and downstream processing steps.